At present, in concrete molding products utilized, for example, in hume pipes or box culverts, chemical prestress has been introduced by incorporation of expansive additive for improving the flexural strength thereof (Articles in Seventh Annual Concrete Engineering Lecture Meeting, pp 33–36, 1985).
However, such chemical prestress involves a subject of loss due to creep, drying shrinkage and relaxation of reinforcing bars caused of concretes with lapse of age (“Improvement of Concretes Performance” supervised by Hideyoshi Nagataki, pp 43–46, Published from Gihodo (1997)).
On the other hand, as a method of enhancing the early strength of concrete molding products, autoclave curing (high temperature, high pressure steam curing) has been known. The autoclave curing has a merit capable of obtaining a strength corresponding to about 28 day age in a case of curing underwater at about 20° C. even by curing for about two days for identical concretes and, accordingly, this has been utilized frequently at present industrially, for example, in the production of secondary concrete articles such as concrete piles. Then, since drying shrinkage after the completion of curing is reduced and creep after the completion of curing is also decreased, the autoclave curing is effective in view of the prevention of loss of chemical prestress after the curing also in a case of chemically prestressed concrete molding products.
On the contrary, however, it involves subjects that remarkable shrinkage strain is caused to the components during curing and relaxation of reinforcing bars is increased at high temperature to lose most of chemical prestress (Atsushi Nakamura, Recent Cement, Concrete Products, pp 42–53 Industry and Products No. 53). Therefore, autoclave curing has not been suitable to prestressed components which have to be manufactured by pretension system such as ties in rail roads.
Further, for the curing of the concrete molding products, there can be mentioned high temperature high pressure underwater curing already filed by the present applicant (Japanese Patent Application No. Hei 9-351234). In the high temperature high pressure underwater curing, the pressure in the pressure resistant vessel is increased to about 2.5 to 10 atm, curing water filled in the pressure resistant vessel is kept at a high temperature of about 130 to 180° C., and concrete components are cured being submerged in high temperature high pressure curing water.
However, in the high temperature high pressure underwater curing as described above, the inside of the autoclave apparatus has to be filled with curing water of such an amount as capable of submerging the concrete components, the curing water has to be heated to and kept at about 180° C., in addition, the inside of the pressure resistant vessel has to be depressurized by deaeration for taking out concrete molding products contained at the inside after the completion of curing and a hatch disposed in front of the pressure resistant vessel has to be opened to discharge high temperature high pressure curing water stored inside. That is, on every time concrete molding products are replaced, it is necessary that the inside of the pressure resistant vessel has to be pressurized, a great amount of curing water at high temperature is discharged, then a great amount of water is stored in turn as curing water in the autoclave apparatus and heated to a predetermined high temperature, which have consumed a great amount of water and heat energy to increase the curing cost. Further, since the curing water to be discharged is at an extremely high temperature of about 180° C. or lower, a sufficient care has to be taken for safety insurance.